Gas turbine system embodying rotary positive displacement compressor apparatus



Jan. 18, 1949.

A.UYSHOLM GAS TURBINE SYSTEM EMBODYING ROTARY POSITIVE DISPLACEMENTCOMPRESSOR APPARATUS Original Filed March 28, 1936 3 Sheets-Sheet 1 3 SQ N Q s Q if 3 Q g A a 82 R *1 e Q 1 a x m N Q I}: a 8

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I lgiENTOR. BY

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C Moms! Jan. 18, 1949. A. LYSHOLM 2,459,709

GAS TURBINE SYSTEM EMBODYING ROTARY POSITIVE DISPLACEMENT COMPRESSORAPPARATUS Original Filed March 28, 1956 3 Sheets-Sheet 2 Jan. 18, 1949.LYSHOLM 2,459,709- 7 GAS TURBINE SYS EMBODYING ROTARY POSITIVEDISPLACEMENT COMPRESSORAPPARATUS Original Filed March 28, 1936 3Sheets-Sheet 3 ATTORNEY 2,459,709 UNITED-STATES PATENT OFFICE GASTURBINE SYSTEM EMBODYING RO- TARY POSITIVE DISPLACEMENT COM- PRESSORAPPARATUS I Alf Lysholm, Stockholm, Sweden, assignor, by

Patented Jan. 18, 1949 losses.

mesne assignments, to Jarvis 0. Marble, Leslie M. Merrill, and Percy H.Batten, as trustees Au ust 7 Claim.

In the case of .a gas turbine plant where the compressor is driven by aturbine which is mechanically independent of the turbine producing powerfor external use, the compressor capacity may be controlled within widelimits by changing the speed of operation of the compressorindependently of the speed of operation of the use- 25, 1943, Serial No.500,010

In Sweden March 28, 1935 ful power turbine, the latter, particularly inturbine. and if the useful power turbine cannot be operated at variablespeed because of the character of the power output therefrom. In

the latter instance, the quantity of air delivered by the compressor canbe controlled by throttling to compensate for varying loads. This kindof control is however undesirable because it introduces into the systemunavoidable throttling It has also been proposed heretofore to admitsuction air to different stages of rotary compressors in a manner suchthat the suction air is admitted to a pressure stage higher than thenormal low pressure stage when the load on the plant, andconsequently'the required output from the compressor, is reduced. Withthis method also, the emciency of the compressor drops materially fromits normal efllciency when the compressor is operated under loads mate--rially different from the normal load for which the compressor isdesigned. Control of the compressor apparatus in order to take care ofvariable loads has also been suggested, in which the work of compressionis divided up among a number of independently operable compressors, oneor more of which may be disconnected, depending upon the amount ofchange in the load imposed on the system. This last-mentioned method ofcontrol is eflective only to a certain extent, since by changing thenumber of compressors in operation it is possible to effect only a verycoarse control of the quantity or pressure of the air delivered and inorder to obtain the requisite nicety of control, resort must be had inaddition to throttling, which involves the introduction of ltheundesirable and unavoidable throttling osses.

Amongst the major objects of the present inventionare: to provideimproved compressor apparatus capable of operating with acceptableefficiency over a wide load range while at the same time operating atconstant speed or with variations in speed which are relatively minor ascompared with the degree of variation of the load; to provide improvedcompressor apparatus of the screw type in which variable output may beobtained at constant speed of operation without involving undue losses;to provide improved screw compressor apparatus in which variations incapacity of the apparatus are obtained at constant speed by means ofnovel bleeding 'arrangements which permit a desired quantity of air tobe bled from the compression spaces of the compressor before compressioncommences; and to provide improved gas turbine system apparatus of thecontinuous combustion type embodying compressor apparatus and controlstherefor enabling the system to be operated efflciently at variable loadand withconstant or substantially constant speed of operation of thecompressor apparatus constituting a part of the system. Y

For a better understanding of the nature of the present invention andthe advantages to be derived from its use, reference may best be had tothe ensuing portion of this specification in which is described severalembodiments of apparatus for carrying the invention into eflect and tothe accompanying drawings forming a part hereof, in which suchembodiments are illustrated.

In the drawings:

Fig. 1 shows more or or less diagrammatically in longitudinal crosssection a gas turbine system of the continuous combustion typecomprising apparatus embodying the invention;

Fig, 2 is a section similar to Fig. 1 showing another arrangement ofapparatus embodying the invention;

Fig. 3 is a transverse section through compressor apparatus having handoperated bleeding means embodying the invention;

Fig. 4 is a longitudinal section partly in elevation of another form ofcompressor apparatus embodying the invention, the section being taken onthe line 4-4 of Fig. 5;

Fig. 5 is a sectiontaken on the line 5-4 of Pig.

Fig. 6 is a plan view of the compressor shown in Fig. 4; 1

Fig. 7 is a view similar to Fig. 6 but with a portion of the compressorcasing broken away and in section on the line 1-1 of Fig. and

Fig. 8 is a fragmentary section similar to Fig. 5 of a modification ofthe compressor apparatus shown in Figs. 4 to 7.

Referring now more particularly to Fig. 1, it designates generally adouble rotation radial flow gas turbine of known kind having shafts l2and i4 upon which are mounted the armatures of electric generators l6and i8. The turbine and generators are mounted in a common frame orcasing-20 having bearings 22, 24, 26 and 28. Shaft i2 is arranged todrive a compressor 3 0 and shaft l4 drives a compressor 32.

Compressor 30 is of the screw type and, in the embodiment illustrated,comprises two rotors 34 and 36 mounted in suitable bearings in housing38. The rotors are preferably space packed. That is, clearance isprovided between the rotors and the walls of the casing and between theintermeshing portions of the rotors. The rotor construction mayadvantageously be in" accordance with the principles disclosed in myco-pending application, Serial No. 44,935, filed October 14, 1935, nowPatent No.'2,243,8 74. Because of the utilization of space packing, therotors may be operated at the very high speed resulting from directconnection of the rotors to the turbine. This connection is made throughthe coupling 40 connecting shaft i2 with the shaft part 42 of rotor 34.The gear 44, fixed on the rotor shaft 42 meshes with gear 48 fixed tothe shaft part 48 of the rotor 36 and provides drive for the latterrotor and maintains the peripherally spaced relation of the intermeshingparts of the two rotors. Rotor 36 is provided with a pressure equalizingpiston 50 for equalizing axial pressure. If the shaft l2 of the turbineis assumed to rotate clockwise when viewed from the left, air which isdrawn in through the inlet openings 52 and 54 is-compressed between thelobes of the rotors forming working spaces the volumes of which diminishtowards the right hand side of the compressor. The compressed air entersthe outlet 56 on that side of the compressor which lies in front of theplane of the drawing and which may be termed the pressure side of thecompressor as distinguished from the suction side located behind theplane of the drawing. In order to prevent direct communication betweenthe inlet and the outlet of the compressor, the end surfaces at theright of the rotors are partly closed by an end wall member 49 which onits front side is pro- -vided with an opening SI for the flowtherethrough of the compressed air, The compressor casing 38 is providedwith a lateral opening 58 providing a cylinder for a valve member suchas the plunger 60 which when it is in the position indicated in thefigure closes the opening 58; An annular space 62 communicating with anoutlet 64 provides communication between the interior of the compressorcasing and the atmosphere when the valve member 80 moves outwardlybeyond the space 62. It will thus be evident that the opening affordedat 64, which may conveniently be termed a bleeder opening, may be if thebleeder valve is opened, compression in such spaces will commence onlyafter the time when the thread or lobe limiting the of the valve isflush with the inner surface of the casing andpreferably the inner faceof the valve is made to conform to the curvature of the casing. Thereason for this is to avoid any large leakage space at this point,through which the air being compressed in one compression space may leakpast the posterior limiting lobe, to the next succeeding space on theother side of such lobe.

The bleeder valve member is provided with a'stem 66 forming the armatureof a solenoid 6d and is also encircled by a spring 10 located betweenthe housing and an abutment plate 12. Spring 10 tends to move the valvemember to open position and this tendency is opposed by the action ofthe solenoid winding, which when fully energized has sufficient strengthto overcome the action of the spring and maintain the valve member inthe closed position shown in the drawing. Solenoid 68 is energized bycurrent supplied through wires 14 and 16 from the generator mainsindicated generally at 18.

The outlet 56 of the compressor 30 is connected by means oLconduit to acombustion chamber 82 having a combustion space 84' surrounded by ajacket 86. Fuel such for example as oil is supplied by a pump, such asthat indicated at 88, to the burner nozzles indicated at and- 92. Agovernor 94 operated from the shaft i4- actuates a fuel control member96 to control the amount of fuel fed to the combustion chamber underdifferent conditions of load. A pressure relief valve 9| .connected tothe discharge pipe of the pump 88 permits return of fuel oil to thesupply pipe of the pump through suction side in front ofthe plane of thedrawing,

brought into full or partial communication with is connected by means ofcoupling 98 to shaft i4 and is similar in construction to the compressor30 previously described and need not be described in detail. Compressor32 has air inlet openings at Hill and H32 and is provided with a bleedervalve I04 urged toward open position by spring I06 and closed by thesolenoid I08, the latter being energized by current derived throughwires H0 and H2 connected into the electrical system 18. The outlet 4 ofcompressor 32 is connected by means of conduit H6 to the combustionchamber 82.

Insofar as air flow is concerned, it will be seen that compressors 30and 32 are arranged in parallel, both delivering to the outer chamber ofthe combustion apparatus, part of the air entering the inner chamber 84to supply the oxygen necessary for combustion of the fuel and partflowing through the jacket space around the inner chamber. Motive fluidconsisting of the combustion gases from the inner chamber and thesuitable factor which changes assavoo 5 Iacketing air is conductedthrough the twin pipes H8 and I2II to the turbine III for expansion inthe turbine.

As indicated. the generators I8 and I8 feed into a common electricalsystem and, as is usual in cases of this kind, operate synchronously.

With normal load on the system, the requisite quantity of f el is pumpedto the combustion chamber and with the motive fluid generated therein bycombustion of the fuel with the air compressed in the compressors, theturbine generates the power required to operate the generators and alsothe power required to in addition drive the compressors. Under suchnormal load conditions, the bleeder valves of the two compressors areclosed and the compressors operate under what may be termed full loadcompression.

When the load on the plant changes, the fuel supply is governed so thatan increased quantity of fuel is supplied upon increased load and a,reduced quantity of fuel is supplied with decreasing load. Assumingconstant normal full load, the quantity of air delivered by thecompressor remains constant. Upondecrease in load, the closing eifectexerted by the sole'noids which hold the bleeder valves closed isdecreased and these valves will open to an extent governed by the degreeof the reduction in the load, so' as to open more or less the bleederopenings. Consequently. the greater the decrease in load on the plant,the greater will be the quantity of air which is discharged through thebleeder openings and the smaller will be the quantity of air which iscompressed in the compressorsto the final pressure for delivery to thecombustion chamber. When the bleeder valves are opened, the finalpressure of the compressed air as well as the quantity of air compressedper unit of time will be reduced, because the ratio of the volume of thecompression spaces at the commencement of the compression period to thevolume thereof at the end of the compression period is constant. If aconstant final pressure is desired regardless of variations in load orif the final pressure is desired to be varied with variations in load ina manner other than that effected with this apparatus, this may beaccomplished by means to be hereinafter described and adapted to varythe ratio of compression space volume at the commencement and end of thecompression periods.

The bleeder valve control need not necessarily be made dependent uponthe electrical load on the plant but may be controlled by any otherdicative of changes in the value of the load.

with and is incompressors is expanded in the turbine to pro duce thepower required for driving the generator and the compressors.

The low pressure compressor I28 is provided with a bleeder opening 84and a bleeder valve 88 controlling this opening, the construction beingsimilar to that previously described with reference to Fig. 1. In thisinstance, however, the control of the bleeder valve is different fromthat previously described. In this embodiment the stem 88 of the bleedervalve is provided with a piston I88 working with a tight fit in cylinderI81 and acted on by a spring 18 tending to move the piston and bleedervalve to a position in which the bleeder passage is opened. The upperside of piston I88 is placed in communication with the compressed airconduit I42 by means of the pipe I44. A

The hydraulic coupling I28 is placed in communication with a governorindicated generally, at I48 by means of a pipe I48. The governorcomprises two plungers I80 and I82 connected to each other and workingin a cylinder I84. A piston [88 is rigidly connected to the plungers I58and I82 and works in a cylinder I88. .4. spring I88 acts on one side ofpiston I88 while the other side of the piston is exposed to the pressureexisting in conduit I42 and pipe I44 by means of the connection I82.Pipes I84 and I88 connect the interior of the governor with a liquidsupply vessel I88. The vessel I88 is located at a lower level than thatof the hydraulic coupling so that the working liquid may under certainconditions of operation flow by gravity from the couplingto the supplyvessel. A gear pum-p I18 is located in the pipe I84, the suction sidebeing connected to the supply vessel and the discharge side to thegovernor.

The pump feeds working liquid in the direction of the arrow I12. Apressure relief valve I14, on the discharge side of pump I18, permitsrea turn of working liquid from the pump to the An example of such othercontrol is illustrated in the plant shown in Fig. 2. In this embodiment.the gas. turbine Illa is indicated as being ofthe axial'flow typeconnected for direct drive of a generator I22 which delivers the poweroutput of the system. Turbine ,I8a also drives directly a compressor I24similar in type to the compressors previously described except for theomission of the bleeder valve arrangement shown on the compressorillustrated in Fig. 1. Compressor I 24 supply vessel through the returnpipe I18 when the discharge end of pipe I84 is closed by the governor.

Operation of the system is as follows. Under full load operatingconditions, the bleeder valve 88 is closed, as shown in the drawing, thestrength of spring 18 being such that the spring is compressed byapplication of normal full load air pressure to the upper side of pistonI88. Governor I48, under the assumed full load condition, is as shown inthe drawing, with the pipes I84 and I48 connected and with the inlet endof pipe I88 closed. The working chamber of the hydraulic coupling isfilled with working fluid and the two compressors are connected so thatboth are operated by the tu'rbine. If it is now assumed that the load onthe generator is reduced, the fuel supply is correspondingly reducedthrough the action of the governor and proportionately lower amount offuel as compared with air supplied to the combustion chamber. Thereduced temperature of the motive fluid results in a reduction in thepressure prevailing in the combustion chamber and in the pipe I42communicating therewith. This will be easily understood 'from thefollowing calculation. The quantity of motive fluid flowing through aturbine can be expressed by the equation:

where F isthe flow of motive fluid in pounds per unit of time, P theinitial absolute pressure of the motive fluid in pounds per square inch,1: the specific volume of the motive fluid at the pressure P, and K aconstant. The value of u may be derived from the well-knownthermodynamic equation Pv=BT, in which T is the absolute temperature andB a constant. Substituting the value of v in the first equation,

I K K At the first moments after reduction of the fuel supply, theamount of motive fluid flowing through the turbine is approximately thesame as before, the reduction in quantity due to the decreased fuelsupply being of negligible order as compared with the quantity of airdelivered by the compressor. Thus, if P and T represent initial pressureand initial temperature respectively at normal load, and P1 and T1represent initial pres-' sure and initial temperature respectivelyshortly upon reduction of the fuel supply,.

For instance, if at normal full load P=80 pounds per square inch, T:1800degrees F. absolute, and if the temperature due to the reduced fuelsupply is decreased to Ti=l600, the pressure in the combustion chamberand in the pipe I42 will immediately be reduced to 11650 Pl-80 I Due tothis reduction in the pressure the bleeder valve 60 commences to openunder the influence of spring III. This opening or partial opening ofthe bleeder valve 60 permits air to flow to a greater or lesser extentthrough the bleeder opening 64 to atmosphere. Thus, a reduced quantityof air, corresponding to the reduced load and the reduced fuel supply,will be com: pressed and supplied to the combustion chamber. Reductionof the quantity of air compressed will tend to bring the temperature ofthe motive fluid back to about normal value, the quantity of motivefluid produced per unit of time now being reduced as compared with fullload conditions and the pressure of the motive fluid being reduced alsobecause of the reduced pressure at which the air is finally deliveredfrom the compressor system because of the bleeding of some =75.4 poundsper square inch of the air. It will be seen that for every value of partload, there is a definite position of the bleeder valve 50 andconsequently for each value of part load, there is a definite quantityof air discharged. from the bleeder opening.

As the load on the system falls below the normal load value and as aconsequence the pressure in conduit I42 is reduced, the pressure actingon piston I56 of the governor will also be reduced. This piston'thencommences to move toward the right, as viewed in the figure, under theinfluence of spring I60 and this movement is transmitted to the rightfrom the position shown in the figure will have no efiect on thehydrauliccoupling but after the loadand the'pressure oi the motive fluidhave fallen below a predetermined value of part load, for example,idling load, the plunger I50 will close the outlet of pipe I64 andsimultaneously the plunger I52 will open the inlet of pipe I66. Movementofrthe governor to this position closes the connection between pump I70and the coupling, which is instead connected through pipes I08 and N0 tothe supply vessel I68 to which the working fluid in the coupling willflow by gravity, in the direction indicated by the arrow H8. Draining oithe working fluid from the hydraulic coupling renders this couplinginoperative and the compressor i28 ceases to operate. At this time aspring loaded check valve I80 in the air conduit I36 connecting the twocompressors is opened by the suction created by the compressor I24 andair is now compressed only in this compressor.

If the load increases from the value at which compressor i28 isrenderedinoperative, the accompanying increase in the amount of fuelsupplied will result in rising temperature and pressure of the motivefluid. The increased pressure of the motive fluid acting on piston I56of the governor moves the plunger valves of the latter to the left andwhen a predetermined value of load is reached, the original connectionsare estab-.

' lished, that is, the pump is agam connected with the hydrauliccoupling and the drain pipe I66 is closed. The pump refills the couplingwith working liquid and the low pressure compressor I28 is brought backinto operation. Reestablished operation of the low pressure compressorproduces pressure in the connecting conduit I36 and as a consequencevalve I80 closes. With this valve closed, the compressors again operatein series. Further increase in load causes the bleeder valve 60 to bebrought more and more nearlyto closed position, progressively cuttingdown the amount of air bled from the low pressure compressor and, whennormal full load has been reached, the

4 parts are again in the position shown in the draw- Whiieautomaticcontrol of the bleeder valve or valves is advantageously employed,manual control may be resorted to and in Fig. 3 a manually controlledvalve is illustrated. In. this figure, which shows a compressor intransverse section,

rotors I82 and I86 operate in the direction indicated by arrows I86 andI88 in casing I90. The

' casing I90 is provided with a bleeder opening I92 at the lower end ofa cylinder I94 in which the bleeder valve I96 of the plunger typeoperates.

Bleeder valve I96 is adapted to seat against the casing in the closedposition of the valve shown in the figures and is advantageouslyprovided with an extension or plug portion I96a, the lower surface ofwhich is curved to form a continuation of the cylindrical inner surface,of the casing. A space I98 surrounding the valve communicates with thebleeder outlet passage 200. A stem 202 is attached to the valveand isprovided with a key 202a working in a suitable slot in the yoke 206 toprevent rotation of the bleeder valve. The

to the plungers I50 and I52. Initial movement upper portion of the stemis threaded at 206 through the hub of a hand wheel 208, the rotation ofwhich on the threaded stem acts to lift the valve.

In the position of the apparatus shown, the bleeder valve is closed andcompression of trapped air takes place in the compression space 2I0between the threads or lobes 2 i 2 and 2 I4 of the rotor I82. Space 2I0extends helically toward the suction side of the compressor to a pointwhere a cooperating thread or lobe of the rotor 34 pro- Jects into thespace so as to progressively reduce its volume upon rotation of therotors in the directions indicated. The succeeding compression space2l8, looking in the direction of rotation of the rotor, is still incommunication with the suction side and remainsso until the edge 2|8 ofthe lobe 2l2 reaches the left hand limiting edge 220 of the bleederopening. At this moment, compression commences in space 218. If thebleeder valve I" is then opened, air may escape through,

the bleeder opening without itshaving been appreciably compressed. Fromthe foregoingit follows that, for instance, space 2 i ii is incommunication with the bleeder valve I98 during the period from theinstant the edge 2i! of the lobe 2 reaches the left hand limiting edge220 of'the bleeder opening to the instant the edge 2l8 of the lobe 2 l2reaches the right hand limiting edge 222 of the bleeder opening.

Advantageously the bleeder opening I92 is shaped so that communicationof the compression space with the atmosphere takes place with minimumthrottling losses. To this end, it is preferable to make the limitingedges 220 and 222 of the bleeder opening parallel, or approximatelyparallel, to the helical edge 2l 8 of lobe H2 and consequently in thesame relation with respect to the corresponding edges of theremaininglobes of the rotor. With the limiting edges of the bleeder openingparallel to the outer edges of the rotor lobes, maximum speed of openingand closing a: the bleeder opening with respect to the compressionspaces, will be obtained.

'10 This slide isaxially movable as indicated in Fig. 4, through themedium of a control rod 236 which may be either manually operated oroperated automatically in response to load variations on the system inwhich the compressor is incorporated, as has already been described inconnection with Figs. 1 and 2'. k y

The end of the slide" at the inlet end of the compressor is preferablyprovided with curved edges 228 and 240 adapted to-abut againstcorrespondingly curved edges 242 and 244'when the slide is moved to theright hand end of its travel as seen in the drawings. At the outlet endof the compressor, the slide is advantageously provided with curvededges 248 and 248; these latter edges determining the area for radialexhaust from the compression spaces and consequently I affecting thetotal exhaust area from these spaces and the discharge pressure of thecompressed air. If we now assume the slide to be moved to the extremeright hand position with edges 228 and 240 abutting respectively againstedges 242 and 244, compression will commence at the moment intended fornormalfull load compression. This will be at the time when the edges ofthe rotor lobes, which cooperate to form a given compression space, passthe limiting edges of the inlet ports.

By making the lower surface of the bleeder valve curved to conform tothe curvature of the inner surface of the casing, leakage back from thecompression space 2lli to the space 2l8 is prevented when the edge 218passes the bleeder opening I92 under full load operating conditions whenthe bleeder valve is closed.

As previously mentioned, the relation of the variation in finalcompression pressure to the variation in the quantity of air compressedat part loads can be varied as desired by changing the compression ratiowhich is affected under different conditions of load.

Apparatus for accomplishing this is shown in Figs. 4 to 7.

Referring now to these figures, the compressor comprises a casing 224 inwhich rotors 22! and 228 are mounted, these rotors being geared togetheras in the compressors previously described and being adapted to bedriven by means of a power input shaft 220. In this embodiment, six

lobes are shown on one rotor and seven on the other in contrast to thethree lobes shown for example in Fig. 3. It will be understood that thenumber of lobes per rotor may be varied as.desired within the scope ofthe present invention.

The casing 224 is provided with inlet openings 22! and .221 and anoutlet 222. In this instance, it will be observed that the inlet portsin the casing provide for both radial and axial admission of air to thecompression spaces and both radial and If it is desired to decrease thequantity of air delivered by the compressor, the control member or slide234 is moved to the left from its extreme right hand position, to aposition such for example as that shown in Fig. 7 and this movementopens up passages 254 and 2" between the end tion. commencement ofcompression in any given compression space will be delayed until theedges of'the lobes defining the posterior boundary of such space passbeneath the slide edges 23! and 240. 4

If for the moment we ,disregard the effect on the outlet opening of themovement of the slide to the left from its full load position, it willbe evident that the compression ratio will be altered by such movementto the left since the volume of the compression space at the momentcompression commences is decreased .as compared with full loadadjustment, while the volume of the compression space at the moment whenthe exhaust port opens remains constant. This would result in decreasedpressure at the moment of discharge from the compressor space.

Obviously however, movement of the control slide toward the left to apart load position will cause the area of the outlet opening to bedecreased and, consequently, as the volume of the compression space atthe instant compression commences is: decreased by movement of theslide, the volume when exhaust occurs is decreased and the finalcompression volume is also decreased. It will be evident that the changein the relation of the volume of the compression space at the momentwhen compression commences to the volume of the compression space at themoment when exhaust or discharge come mences can be varied as desired bysuitably relating the contours of the edges of the slide which controlthe commencement ofcompression and the termination of compression in thevarious positions of the slide;

Fig. 8 shows a modification of the apparatus illustrated in Figs. 4 to7. The construction is similar to that described above except the formof the control member or slide which in this instance is in the iorm ofa cylinder 280 the lower portion of which is cut off and connected witha member 262 the inner surface of which is formed to the same radius asthe radii of the rotors. As in the example previously described, thecontrol member is axially movable by means of a control rod 284. As willbe easily'understood from the drawing, the shape of the control mem ber260 facilitates manufacturing of the apparatus as compared with theshape of the control member according to Figs. 4 to '7.

From the foregoing description, it will be evi dent that many specificdifferent forms of structure may be employed within the scope Of theinvention and it is to be understood that the invention embraces allthat falls within the scope of the appended claims.

I claim:

I L-In a gas turbine system of the continuous combustion type, means forcompressing air to be used as a constituent of motive fluid forexpansion in the system comprising a rotary positive displacementcompressor, means for bleeding the compression spaces of said compressorto retard the time of commencement of compression in said spaces untiltheir volume has been reduced to less than their normal maximum volume,and means responsive to variations in the load on the system forcontrolling the flow ofair through said bleeding means.

2. A system as set forth. in claim 1 in which said compressor is of thetype having inter-meshing screw wheel rotors and in which said bleedingmeans comprises valve means carried by the compressor casing to vent thecompressor spaces intermediate the ends of the rotors and movableresponsive to said control means to delay commencement of compressionand to reduce the quantity of air compressed in each of said spaces byretarding the time of closing of the spaces.

3. A system as set forth in claim 1 in which said bleeding means isresponsive to the pressure of the air delivered by the air-compressingmeans and is arranged to bleed the air in accordance with predetermineddrop in load on the system.

4. A system as set forth in claim 1 in which the means for compressingair comprises a plurality of compressors arranged in series and in whichthe bleeding means is associated with the low-pressure compressorwhereby to control the quantity of air compressed inthe system.

5. A system as set forth in claim 1 in which the means for compressingair includes a plurality of serially connected compressors with saidbleeding means associated with the low-pressure compressor, couplingmeans for disconnecting I the low-pressure compressor and meansresponsive to variations in load on the system for disconnecting thelow-pressure compressor when the load on the system drops below apredetermined value.

6. A system as set 'forth in claim 1 in which the means for compressingair includes a plurality oi serially connected compressors with saidbleed= ing means associated with the low-pressure compressor, couplingmeans for disconnecting the low-pressure compressor, and meansresponsive to variations in load on the system for disconnect- LYSHOLM.

REFERENCES CITED The following'references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,584,602 Bradshaw et al. May 11,1926 2,048,566 Rosch July 21, 1936 2,110,714 Place Mar. 8, 1938 FOREIGNPATENTS Number Country Date 418,463 Great Britain Sept. 14, 1934

